This application is a continuation of PCT/US00/22723, filed Aug. 18, 2000, which claims the benefit of U.S. Provisional application 60/150,068, filed Aug. 20, 1999, which are incorporated herein by reference as if fully set forth.
The present invention relates to papermaking fabrics, especially dryer fabrics. More specifically it relates to fabrics made from interconnected modular subassemblies. Most specifically it relates to pre-molded subassembly links used to make a modular fabric.
A papermaking fabric is used in the form of an endless belt which is supported by and advanced through the papermaking machine by various machine rolls. The process and the various sections of the machine, forming, press and dryer, will be known to those skilled in the art.
Traditionally fabrics have been made either through endless or flat weaving techniques. More recently, spiral fabrics have been made by connecting spiral coils with pintles to create a fabric. The spiral fabrics have allowed for greater flexibility in making fabrics of various dimensions, unlike flat or endless woven fabrics whose dimensions must be known ahead of time and are limited by loom design. The spiral fabric, however, lacks adaptability with regard to desired changes in drainage, permeability and surface characteristics.
Papermaking fabrics, especially dryer fabrics, commonly comprise woven monofilament yams. The monofilaments have traditionally been extruded from materials such as nylon, polyester, etc. Unfortunately, the extrusion process renders many plastics unsuitable for use in the harsh environment of the paper machine""s dryer section.
Therefore, the choice of materials suitable for use in forming the monofilament has been limited. Many more plastics would become available if a dryer fabric could be made with molding techniques. To date, few practical mechanisms exist for constructing fabrics from molded parts.
One prior attempt at forming a dryer fabric for a paper machine from molded components is described in DE 37 35 709 A1. This reference discloses flat plastic elements which are interconnected by pintles or articulated joints, with the spacing of the elements and the size of the apertures therethrough being selected to provide a desired air permeability for the fabric. However, each of the molded components extends across an entire width of the fabric and there is no teaching of the necessary features to successfully practice the invention in connection with commercial papermaking dryer fabrics, which typically have a width of 10 meters (30 feet). There is also no suggestion as to how such molded components, which extend across an entire fabric could be economically manufactured and assembled, or of molded subassemblies having a width smaller than the entire fabric width or a manufacturable aspect ratios and thicknesses for such subassemblies which can be assembled together to form a dryer fabric. Additionally, this references teaches punched or stamped through openings which are formed in the flat elements or the fabric after it is assembled. This introduces additional cost as well as increased potential for damage to the pintles.
U.S. Pat. No. 4,842,905 discloses a non-woven papermaking fabric assembled from elements interconnected by a complementary geometric shapes in a tessellation (i.e. constructed in the style of a checkered mosaic). Standardized xe2x80x9cjig-saw puzzlexe2x80x9d type interlocking elements which are interconnected along their longitudinal edges by integral male members or projections along a first longitudinal edge of one element being engaged with female members or recesses located along the second longitudinal edge of the adjacent member. These complementary projections and recesses are interlocked under normal machine tensions. However, such arrangements have not been applied in practice since positive connection of such elements cannot be guaranteed, and in paper machine applications, papermaking fabrics must operate continuously for months without failure of the connections holding the fabric together. There is also no suggestion of the thickness and tensile strength requirements needed in order to realize a dryer fabric utilizing such elements.
U.S. Pat. No. 4,537,658 also discloses a fabric comprised of generally rectangular, elongate elements whose lengths are equal to the finished fabric width. The elements are provided with generally xe2x80x9ctxe2x80x9d shaped slots extending completely along one longitudinal edge through which a pintle is inserted to interconnect the elements to form the fabric. The pintle itself is required to have a shape that is complementary to the shape of the slots in the elements and must be glued, welded, or bolted in position so that it is retained within the fabric. These elements are extruded, and can not be economically molded based on their size, and any apertures would have to be formed in a separate operation. Additionally, there is no suggestion of thickness and tensile strength requirements for forming such a fabric which can be manufactured economically and used as a dryer fabric.
Present dryer fabrics form endless belts passing around rollers having diameters from 18 to 60 in. (45.7 to 152.4 cm). While flexibility is an important requirement, fabrics also must be strong enough to support the paper web along its path under a variety of conditions and temperatures. Suggested load capacities have been fifteen pounds per linear inch (PLI) (267.9 kg/m). The fabric must also withstand traveling at greater than 4,000 feet per minute (1219.2 m/min).
Damage and dirt accumulation are also major factors which typically limit the maximum useful life of the fabric to about one year. Fabric edges are particularly vulnerable because of a tendency of the yams to unravel and shift. Once damaged, the entire fabric must be replaced. Although traditional woven fabrics have been limited in size by loom construction, they have still reached as much as thirty feet wide by three hundred feet long. Damage to even a small area of the fabric necessitates costly replacement of the entire fabric.
Even minor marring of the surface may deteriorate fabric quality because the paper contact surface characteristics greatly affect the final paper product. Traditional fabrics adjust these characteristics through choice of materials and the type of weave used. Often, a compromise between the best material or the best weave and final product quality must be made. Batting or other material has been affixed to the paper support surface to gain benefits not available from standard materials and weaves. A molded fabric offers greater flexibility in this regard, as surface characteristics may be incorporated directly into the mold and repeated consistently throughout the fabric.
The use of molded fabrics will benefit the art in many ways. A more direct process, avoiding additional storage and coiling requirements of monofilament yams, as well as reducing trimming time and eliminating sealing will be enjoyed by using molded fabrics. More choices of less expensive material will become available, including lower molecular weight materials and gels having less stringent filtration requirements. The molding process also allows the use of composite materials to achieve more beneficial physical properties while maintaining cost effectiveness. A molded fabric allows greater flexibility and efficiency in design when creating fabric patterns (i.e., weave patterns and fabric dimensions). A fabric assembled from pre-molded subassemblies is strong, dimensionally stable, thermally stable, easy to join, distortion free, and has tough finished edges. Furthermore, use of a molded fabric limits fabric stretch, reduces costs, facilitates repair and generally benefits the papermakers art.
The present invention is a pre-molded plastic subassembly for making papermaking fabrics. A plurality of the subassemblies are interconnected to create an endless fabric. The completed fabric also forms a part of the present invention.